Tensile Deformation and Fracture Mechanism of Bulk Bimodal Ultrafine-Grained Al-Mg Alloy
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The tensile fractures of ultrafine-grained (UFG) Al-Mg alloy with a bimodal grain size were investigated at the micro- and macroscale using transmission electron microscopy (TEM), scanning electron microscopy (SEM) equipped with focused ion beam (FIB), and optical microscopy. The nanoscale voids and crack behaviors near the tensile fracture surfaces were revealed in various scale ranges and provided the evidence to determine the underlying tensile deformation and fracture mechanisms associated with the bulk bimodal metals. The bimodal grain structures exhibit unusual deformation and fracture mechanisms similar to ductile-phase toughening of brittle materials. The ductile coarse grains in the UFG matrix effectively impede propagation of microcracks, resulting in enhanced ductility and toughness while retaining high strength. In view of the observations collected, we propose a descriptive model for tensile deformation and fracture of bimodal UFG metals.
Structural nanocrystalline (NC) and ultrafine-grained (UFG) metals possess remarkably high strength, but generally suffer from low ductility and toughness.[1,2] The deterioration of ductility and toughness presently limits the use of UFG metals in manufacturing bulk mechanical parts. This is a major barrier to the widespread use of these materials.
In an effort to enhance the ductility and toughness of bulk UFG metals, incorporating coarser grains (CGs) in a UFG matrix has been suggested as a means of overcoming the observed brittle behavior.[3, 4, 5, 6] The motivation was based on the hypothesis that if a small proportion of CG material was added to the UFG matrix, the ductility could be increased with only a small decrement in strength analogous to ductile-phase toughening. Thus, as a simplest case, a bimodal grain size distribution, encompassing both NC and UFG regimes, has been pursued in an attempt to exploit the advantages of both increased strength resulting from grain refinement and retention of substantial ductility resulting from incorporation of ductile CG.
In one recent study, high tensile ductility was achieved in annealed NC Cu with a bimodal grain size in the NC regime. In an earlier study of UFG alloys, Tellkamp et al. reported tensile elongation of over 8 pct in a cryomilled bulk Al alloy without significant loss of strength. The authors suggested that the presence of CG material in the NC or UFG matrix might be responsible for the enhanced ductility. Building on this early work on cryomilled Al alloys, Witkin et al. demonstrated a more feasible method to achieve a bulk bimodal microstructure from cryomilled powder by design. The deliberate blending of unmilled CG powders and cryomilled NC/UFG powders resulted in a bimodal grain structure comprised of a hard UFG matrix with ductile CG inclusions. This powder metallurgical process route to manufacture bimodal structures allows the convenient combination of constituents of different strength and ductility without the compositional differences normally associated with composite materials. Thus, a bimodal Al-7.5Mg consisting of UFG and CG constituents yields balanced mechanical properties that include enhanced yield and ultimate strength and acceptable or superior ductility and toughness compared to conventional grain-sized alloys and UFG metals only.
However, the underlying deformation and fracture mechanisms associated with bulk bimodal metals, which can render them attractive for further design, have not been fully elucidated because of a lack of unambiguous evidence and direct observations in various scale ranges.[6,9, 10, 11, 12] The major difficulty in direct cross-sectional observation of tensile fractures of UFG Al alloys stems from the sample preparation of near-fracture surfaces, which can preserve embedded voids and cracks, for scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the present study, the surface treatment for SEM and thin foil preparation for cross-sectional TEM are facilitated by focused ion beam (FIB), which preserves embedded voids and cracks for systematic scrutiny. Cross sections of tensile fractures were examined to determine the deformation and failure mechanisms of cryomilled bulk bimodal Al-Mg alloy.
2 Experimental Procedures
The bimodal Al-7.5Mg alloys were produced by mechanical blending of cryomilled UFG powders with 30 vol pct unmilled CG powder in an inert atmosphere to achieve a uniform distribution of unmilled powders. Experimental details on the cryomilling experiments can be found elsewhere.[7,9] For comparison, UFG samples were prepared from 100 vol pct cryomilled powders. The powders used for cryomilling and for unmilled additions were from the same spray-atomized batch. The powder blends were canned and then consolidated by cold isostatic pressing at a pressure of ~400 MPa. The consolidated compacts were vacuum degassed at 673 K. To remove any remaining porosity and improve mechanical properties, the consolidated billets were extruded at 823 K to a round bar 19.05 mm in diameter.
Cylindrical tensile specimens with a gage length of 13.5 mm and a gage diameter of 3 mm were tested on a universal testing machine (Instron 8801, Canton, MA). Uniaxial tensile tests were performed parallel to the extrusion direction at a constant crosshead velocity of 0.012 mm/s until failure, with direct measurement of the displacement of the tensile gage section by a dual-camera video extensometer.
The cross sections of tensile fracture ends were molded and mechanically polished and chemically etched for optical microscopy (OM) observation. The FEI dual beam (FIB) milling on mechanically polished surfaces was used to remove surface oxide and ensure smooth surfaces for SEM observation.
The TEM specimens near tensile fracture surfaces were molded to preserve the fracture surface and cracks and then sectioned with a diamond saw. The thin and small specimens were mechanically polished to a thickness of 2 to 5 μm and bonded to a half-cut TEM slotted grid for FIB milling. Areas of interest beneath fracture surfaces were thinned by FIB milling to prepare electron transparent sections for TEM observation. The TEMs (JEOL1 200CX and field emission PHILIPS2 CM200) were operated at 200 kV for bright- and dark-field imaging.
3 Results and Discussion
The strength and ductility values for bimodal alloys were intermediate between those of the UFG and the all-CG materials, representing a balance of both strength and ductility. This suggests that bimodal alloys have unusual deformation and fracture mechanisms arising from the fine-scale combination of hard and soft phases. The deformation and fracture mechanisms warrant further investigation, which is pursued through examination of tensile fractures.
Direct observations in various scale ranges revealed void nucleation and crack behavior in tensile fracture of bimodal Al-Mg alloys. The bimodal grain structures exhibited unusual deformation and fracture mechanisms similar to ductile-phase toughening of brittle materials. Voids initiated and grew in the UFG matrix and at CG-UFG interfaces. The CG bands tended to deform locally at stress concentrations, arresting cracks by local blunting, resisting crack growth by bridging of crack wakes, and impeding crack propagation by deflecting and branching of crack tips and by delamination during plastic deformation.
The present work provides insights for the design of UFG metals resistant to deformation and fracture. Using these observations, single-phase materials with local variations in grain size can be designed to achieve unique combinations of strength, ductility, and toughness. Using this design approach, the deformation mechanisms can be altered by manipulating (a) the morphology and dispersion of the CG phase and (b) the interface properties and by selecting the intrinsic mechanical properties of phases. Single-phase, composite-like materials, because of the perfect CTE match of the constituent phases, may be well suited to high-temperature applications and processing routes. Further work is warranted to optimize bimodal microstructures for mechanical performance and to extend the approach to different multiscale alloys. Dynamic straining experiments employing in-situ observation techniques should provide further insights into deformation and fracture mechanisms of bimodal materials.
Support from the Office of Naval Research (Contract Nos. ONR00014-03-1-0149 and ONR00014-03-C-0163) is gratefully acknowledged. The National Center for Electron Microscopy is supported by the Director, Office of Science, United States Department of Energy, under Contract No. DE-AC02-05CH11231.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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